Wide-angle planar-beam antenna adapted for conventional or doppler scan using laterally flared reflector

ABSTRACT

An antenna including a conductive reflector formed as a surface of revolution about a horizontal axis by rotation of a doubleended hyperbolic curve. The reflector is illuminated from an ancillary reflector in cooperation with a parallel-plate wave guide planar-beam generating arrangement. A conventional or Doppler-scan array provides the excitation for the so-called parallel-plate wave converter. The radiated beam shape tends to hold its focus and, therefore, its elevation coordinate width over a wide range of azimuth angles.

United States Patent Charlton WIDE-ANGLE PLANAR-BEAM ANTENNA ADAPTED FOR CONVENTIONAL 0R DOPPLER SCAN USlNG LATERALLY FLARED REFLECTOR Inventor: Gregory G. Charlton, Calabasas,

Calif.

Assignee: International Telephone and Telegraph Corporation, New York, N.Y.

Filed: Dec. 11, 1972 Appl. No.: 314,141

U.S. c1. 343/836, 343/108 M, 343/837,

343/854 1111. (:1., lltllg 19 10 Field of Search 343/108 M, 779, 781,

DOPPLER L/NE FEED Dec. 25, 1973 [56] References Cited UNITED STATES PATENTS Primary Examiner-Eli Lieberman AttorneyC. Cornell Remsen et al.

[5 7 ABSTRACT An antenna including a conductive reflector formed as a surface of revolution about a horizontal axis by rotation of a double-ended hyperbolic curve. The reflector is illuminated from an ancillary reflector in cooperation with a parallel-plate wave guide planar-beam generating arrangement. A conventional or Doppler-scan array provides the excitation for the so-called parallelplate wave converter. The radiated beam shape tends to hold its focus and, therefore, its elevation coordinate width over a wide range of azimuth angles.

5 Claims, 3 Drawing Figures REFL E OTOR PARALLELPL/J7'E WA VEGU/DE PATENTEB 3.781.897

REFL E0 TOR DOPPLER L/NE FEED & PARALLEL-PLATE W4 VEGU/DE WIDE-ANGLE PLANAR-BEAM ANTENNA ADAPTED FOR CONVENTIONAL OR DOPPLER SCAN USING LATERALLY FLAREI) REFLECTOR CROSS REFERENCE TO RELATED APPLICATIONS U. S. Pat. application Ser. No. 272,451, filed July 17, 1972, entitled A Technique for Generating Planar Beams From a Linear Doppler Line Source or Linear Phased Array (Jeffrey T. Nemit, inventor) contains disclosure pertinent to the description of the present invention. Accordingly, the disclosure of that application is incorporated, by references, as though fully set forth herein.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to scannable antennas, and in particular, to antenna systems relating to air navigation and guidance systems requiring vertical angle determination and accurately maintained beam shape over a wide azimuth angle.

2. Description of the Prior Art The above referenced U. S. Pat. application is itself descriptive of an improvement in antenna systems known and used in connection with the so-called Doppler-scan technique. The related prior art preceding that invention is typically described and referenced in U. S. Pat. Nos. 3,613,096 and 3,670,338. U. S. Pat. application Ser. No. 210,699, filed Dec. 22, 1971, now U. S. Pat. No. 3,728,729, is also a useful reference for background information in describing the state of the prior art. Those references also are useful in understanding the nature of the problems encountered in Doppler-scan systems, for example. The utility of the present invention is particularly great in connection with systems of those types. I

U. S. Pat. application Ser. No. 272,451 (abovereferenced) describes a device employing a circular aperture, parallel-plate wave guide converter excited by a linear Doppler-scan array or alternatively by an electronic scan phased array or the like. The purpose of that parallel-plate structure, as is fully described in that application Ser. No. 272,451, is the conversion of fundamentally conical-coordinate beams to planarcoordinate beams. That device is employed as an element in the novel combination of the present invention as hereinafter described.

In U. S. Pat. No. 3,653,057, entitled Simplified Multi-Beam Cylindrical Array Antenna with Focused Azimuth Patterns Over a Wide Range of Elevation Angles, a system for tailoring the beam shape of a scanning antenna for wide-angle performance in the nonscanning coordinate is described. That device uses multiple beams to achieve the desired effect, and while satisfactory in typical L-band systems, is relatively inefficient at C-band and above.

The manner in which the present invention builds on the techniques of the prior art and the extent to which it affects improvements thereover will be evident as this description proceeds.

SUMMARY OF THE INVENTION It may be said that the combination of the present invention involves the use of the aforementioned coordinate converting parallel-plate wave guide arrangement of U. S. Pat. application Ser. No. 272,451, as a feed for a surface of revolution reflector formed by rotation of a double-ended, hyperbolic outwardly convex curve at a predetermined radius about a horizontal axis. The combination of that feed arrangement and the reflector described, produces a vertically oriented (functionally) system. That is to say, it is intended to produce vertically scannable beams having predetermined azimuth characteristics which remain substantially constant over the useful elevation scanning angles. The referenced U. S. Pat. No. 3,653,057, on the other hand, describes a horizontally oriented system by the same criterion.

The general objective of the present invention may be said to be the production of a basically vertically oriented antenna (although its use as a horizontally oriented device is not entirely precluded) to form horizontal fan beams over a wide range of azimuth angle and to be adapted for scanning in elevation. The use of a simple linear array does not accomplish this objective because its beam has a conical shape when scanned. The circular parallel-plate wave guide arrangement described in the aforementioned U. S. Ser. No. 272,451 provides the required planar beam for consistency with coordinate systems utilized in the so-called Dopplerscan air navigation and guidance systems. However, the use of that circular aperture parallel-plate wave guide arrangement, as described in the reference, is limited in wide angles coverage (when used by itself) in the nonscanning coordinate, due to defocusing.

By defocusing, as used herein, the following is given for clarification. If it were imagined that an observer from a distance looks down the beam toward the antenna and were able to see the cross-sectional shape of the total beam, it would appear somewhat in the shape of a dog bone. This is because defocusing at azimuth angular extremes tends to flatten the beam in the elevation coordinate. The present invention, which is particularly adapted to use atradar C-band operation and above, employs an optical technique to provide the required wide-angle beam in an arrangement affording the planar coordinates affected by the aforementioned Ser. No. 272,451 device as a feed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partially exploded pictorial view of an antenna arrangement in accordance with the present invention.

FIG. 2 is a top view of the arrangement of FIG. 1 illustrating the distribution of energy into and from the reflector surface to form the required azimuth fan beam.

FIG. 3 is a side view of the arrangement of FIG. 1 showing the relationship of the ancillary reflector more realistically.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I, an overall semi-pictorial view of an antenna system in accordance with the present invention is shown. The so-called parallel-plate wave guide is shown generally at 11. This parallel-plate structure has a circular aperture 14, but is closed along its straight sides, typically 11a.

Excitation of the space between plates of the parallelplate structure is provided typically by a Doppler line feed 10, having a vertically oriented array, in a typical example comprised of slots, such as 18 and 19, in a wave guide structure operating as a feed. This particular structure is described more fully in the aforementioned U. S. Pat. application Ser. No. 272,451.

From the circular aperture 14, the planar beams formed within the parallel plates in accordance with the aforementioned application Ser. No. 272,451, are redirected toward the reflector by an ancillary reflecting device 13. In FIG. 1 this ancillary reflector 13 is illustrated in exploded relationship for clarity. The actual relationship is more readily gleaned from FIG. 3. A satisfactory shape for the element 13 is essentially circular in the plane of the aperture 14 and nearly circular in cross-section. The main reflector consists essentially of two sections, 12 and 12a, but jointed against respective sides of the parallel-plates of 11. It is to be understood that the reflector surface does not continue through the parallel-plate structure, but actually is in the form of two oppositely extending horn-like surfaces. Each of the sections 12 and 12a of the main reflector is formed by rotation of a curve rotated to form a surface of revolution about a horizontal axis Z.

Concerning the formulation of the curve of the reflector sections 12 and 12a, consider the following mathematical relationships. The formulation of the curve is determined by providing the proper slope for the angle of the reflected wave, and by varying the radius to provide for proper phasing at each angle according to:

dR/dZ tan a /2 Equation l R cos 0 R Equation 2 tan a Z/F R Equation 3 where,

R minimum radius of reflector,

R variable radius of reflector,

Z horizontal axis measured from the center (R plane) of the reflector outward,

6 azimuth angle, and

a equivalent feed angle.

From the foregoing, it follows that the phase around any circumference for a given value of Z is determined only by the parallel-plate wave guide section. Equation 2 determines the focus azimuth angle 0 for each R and Equations 1 and 3 provide a description of a surface normal to that angle at each radius R. These equations may be solved digitally using a computer for R as a function of Z, given F and R In accordance with the reflector shape, the required azimuth angle coverage is mainly effected by controlling the reflector slope. The required focusing and e1evation is mainly controlled by the reflector radius. In order to obtain approximately 60 of azimuth coverage, a two-to-one change in radius is required.

Referring now to FIG. 2, a top sectional view taken from FIG. 1 is shown, in order that the distribution of rays from the aperture 14 is shown. The divergence of rays 15, 16 and 17 increases in the outward direction,

as illustrated, in order to provide the said azimuth pattern. Here again, the spacing of the ancillary reflector is somewhat exaggerated for the sake of pictorial clarity. Referring now to FIG. 3, a side or end-on view of P16. 1 is provided and is largely self-explanatory in respect to the location of the various elements, including the ancillary reflector 13.

It will be realized that other expedients are available to perform the function of the ancillary reflector 13. For example, a folded horn, or a dipole array excited from the circular aperture 14 might be employed in order to distribute the energy from the parallel-plate wave guide coordinate converter 11 over the surface of the reflector halves 12 and 12a.

Still further, it is possible to substitute an ancillary reflector of flat cross-section at 13.

It will be realized that elevation beam positioning effected through the parallel-plate wave guide 11 in cooperation with the feed 10 causes the rays, as illustrated in FIG. 2, to travel circumferentially about the main reflector.

Other modifications, in addition to those alternative arrangements in lieu of 13, will suggest themselves to those skilled in the art once the principles of the present invention are understood. Accordingly, it is not intended that the scope of the present invention should be limited by the specific illustrations and description herein.

What is claimed is:

1. An antenna system for radiating planar beams scannable in a first angular coordinate and of substantially constant beam width in a second orthogonal an gular coordinate, comprising:

a main reflector having at least a surface of conductive material, said reflector being in two parts each in the shape of at least a partial surface of revolution about an axis Z, said surface of revolution having a generally horn shape of variable radius R beginning at R, and extending axially one in each Z direction, substantially in a curve on each side of Z= 0 such that the rate of change of R as a function of Z is equal to tan a 0/Z, where a is the equivalent feed angle and 0 is the azimuth angle;

feed means for illuminating said reflector, said feed means comprising a parallel-plate wave guide planar beam converter having a semi-circular aperture and being oriented with said parallel-plate substantially normal to said Z axis, said two parts of said surface of revolution extending oppositely, one from each plate of said parallel-plate wave guide, said parallel-plate wave guide aperture facing in the general direction of formation of said beams;

ancillary reflector means substantially uniformly spaced from said semi-circular aperture in the direction of said beam formation for redirecting said aperture to illuminate said main reflector with the beam formation energy from said parallel-plate wave guide;

and means comprising a linear array having a plurality of antenna elements within said parallel-plates to provide for said beam formation within said parallel-plate wave guide and for scanning of said beam in a plane normal to said Z axis.

2. Apparatus according toclaim 1 in which said ancillary reflector means comprises a conductive surface having a shape curved inwardly toward said main reflector in any plane containing said Z axis, and circucillary reflector curve in said plane containing said Z axis is circular.

5. Apparatus according to claim 2 in which said main reflector partial surface of revolution and said ancillary reflector each extend circumferentially over an are not exceeding 

1. An antenna system for radiating planar beams scannable in a first angular coordinate and of substantially constant beam width in a second orthogonal angular coordinate, comprising: a main reflector having at least a surface of conductive material, said reflector being in two parts each in the shape of at least a partial surface of revolution about an axis Z, said surface of revolution having a generally horn shape of variable radius R beginning at Ro and extending axially one in each Z direction, substantially in a curve on each side of Z 0 such that the rate of change of R as a function of Z is equal to tan Alpha - theta /Z, where Alpha is the equivalent feed angle and theta is the azimuth angle; feed means for illuminating said reflector, said feed means comprising a parallel-plate wave guide planar beam converter having a semi-circular aperture and being oriented with said parallel-plate substantially normal to said Z axis, said two parts of said surface of revolution extending oppositely, one from each plate of said parallel-plate wave guide, said parallel-plate wave guide aperture facing in the general direction of formation of said beams; ancillary reflector means substantially uniformly spaced from said semi-circular aperture in the direction of said beam formation for redirecting said aperture to illuminate said main reflector with the beam formation energy from said parallelplate wave guide; and means comprising a linear array having a plurality of antenna elements within said parallel-plates to provide for said beam formation within said parallel-plate wave guide and for scanning of said beam in a plane normal to said Z axis.
 2. Apparatus according to claim 1 in which said ancillary reflector means comprises a conductive surface having a shape curved inwardly toward said main reflector in any plane containing said Z axis, and circularly curved in a plane perpendicular to said Z axis, said last named curve having a radius greater than the minimum radius of said reflector parts.
 3. Apparatus according to claim 2 in which said ancillary reflector curve in said plane containing said Z axis is hyperbolic in shape.
 4. Apparatus according to claim 3 in which said ancillary reflector curve in said plane containing said Z axis is circular.
 5. Apparatus according to claim 2 in which said main reflector partial surface of revolution and said ancillary reflector each extend circumferentially over an arc not exceeding 180*. 